Internal combustion engines often include cooling systems that provide coolant flow through passages formed in the engine block. The cooling system has a pump to drive coolant flow through the system, and the pump is often mechanically driven by the crankshaft or other rotating component of the engine. The pump used with the cooling system may be a centrifugal pump, which is designed to pump liquids and not gases or vapors. The pump chamber, or volute, needs to be filled or substantially filled with liquid for the pump to operate.
Priming is the act of replacing the air or gas inside the pump volute with coolant, such as liquid water. Often, the pump needs to be primed when it is installed and coolant is first introduced into the cooling system. The pump may also need to be primed at each cold start of the engine as the coolant may drain from the pump volute when the engine is not operating. Generally, centrifugal pumps need to be primed and cannot produce a reduced pressure on the suction side without the presence of liquid inside the volute, or they operate at a reduced pumping efficiency. If the pump chamber contains entrained air or gas, the pump may lose its prime, for example, with the accumulation of air in the eye of the impeller which would block liquid flow through the pump. The performance effect on the pump due to entrained air may vary depending on many variables, including pump operating speed, impeller design, number of vanes, operating point on the curve, suction pressure, etc. Operating the pump for an extended period with entrained air may also cause the pump to operate at a temperature above its normal operating temperature range, and may cause wear or stress on pump surfaces and components. Often the pump is positioned with the discharge port at the high point of the volute to provide venting of gases within the volute.